The present invention is concerned with the recovery of metal values from a variety of chemical operations, e.g., hydroformylation operations employing expensive catalyst metals such as rhodium or cobalt. In addition, the present invention relates to the recovery of metal values from chemical operations employing such metals as platinum, palladium, nickel, or iridium, or other metals complexed with triorganophosphine ligand(s). The gradual oxidation of the triorganophosphine ligand(s) to phosphine oxide(s) which is (are) catalytically inactive material(s), eventually results in levels of such material(s) in the catalyst system or solution as to exert a negative effect on the desired chemical operation.
With particular respect to hydroformylation reactions, the gradual accumulation of high boiling materials such as aldol condensation by-products is inevitable. Excess quantities of such materials must be removed in order to maintain the proper volume or level of the active catalyst system. Moreover, the gradual accumulation of contaminants such as iron salts and pump oils (polyglycol oils) can cause adverse changes in catalyst selectivity, reaction rate, and by-product production.
In addition, hydroformylation operations typically require the addition of make-up triorganophosphine ligand to compensate for ligand which is oxidized to phosphine oxide. Thus, not only does such oxidation effect an additional by-product build-up but also creates a significant financial burden, particularly when the ligand is a costly phosphine derivative.
The removal of these by-products and contaminants is frequently difficult due to the complexity of the catalyst system. The catalyst system contains valuable unassociated triorganophosphine ligand(s) and the active catalyst material such as rhodium complexed with ligand, as well as the by-product contaminants. The loss of substantial amounts of the catalyst metal, in particular, rhodium, in heretofore proposed catalyst system purification procedures has either prevented their commercial adoption altogether or has limited their desirability for use.
A procedure for extracting catalyst metals such as rhodium from hydroformylation operations reported in British Pat. No. 1,502,339 by Tampieri et al. However, the efficiency of the Tampieri process is low and, therefore, not commercially attractive. Tampieri describes a process for the removal of triphenylphosphine oxide from a hydroformylation catalyst system whereby the catalyst system is heated (distilled) to remove the aldehyde product and other volatiles. The residue from the distillation is typically composed of aldol condensation products (15 percent), triphenylphosphine plus triphenylphosphine-rhodium complex (65 percent) and triphenylphosphine oxide (20 percent). This residue is washed with an aqueous solution of methanol, acetone, acetonitrile, or dimethylformamide to give a suspension which separates upon standing at 75 degrees C. This procedure is alleged to remove up to 45 percent of the triphenylphosphine oxide along with about 2.5 percent of the triphenylphosphine-rhodium complex. The phosphine-rhodium complex is recovered by back-washing (extracting) the aqueous medium with a hydrocarbon solvent such as hexane or toluene which extract is concentrated and returned along with the original catalyst to the reactor.